System for dynamic low dose x-ray imaging and tomosynthesis

ABSTRACT

A system for low dose x-ray imaging provides for dynamic generation of an x-ray beam with specific shape, and dynamic tracking of a detector with said beam. The detector is rotatable, and translatable along two orthogonal axes, and may mount with a circular detector tray, the tray rotating around a rotation axis. Specific detector shapes include an elongated rectangular matrix, for example with additional detector cells near the rotation center to provide an increased area of continuous detection. Dynamic low-dose x-ray tomosynthesis or limited-angle tomographic imaging is enabled via simultaneous x-ray tube and detector motions during examination, such as fluoroscopic examination of a human body. Data acquired at multiple projection angles is input to a 3D image reconstruction algorithm that provides a refreshed 3D data set during continuing examination. The system may thus also automatically track a point in three-dimensional space, for example continuously locating the tip of a catheter.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit of priority to U.S. Provisional PatentApplication No. 60/652,127, filed Feb. 11, 2005, and U.S. ProvisionalPatent Application No. 60/654,922, filed Feb. 22, 2005, both of whichare incorporated herein by reference.

BACKGROUND

1. Field of the Invention

This disclosure relates to the field of x-ray imaging, and moreparticularly to the dynamic low-dose imaging of an object or subjectwith a moving detector, as well as to the dynamic low-dose tomosynthesisand limited-angle tomographic imaging of a subject with a movingdetector and a moving x-ray source. Specific applications are in thesub-fields of fluoroscopy, radiography, and cardiology. Otherapplications are in the fields of non-destructive testing, homelandsecurity, and animal imaging.

2. Description of the Related Art

A number of interventional procedures utilize x-ray as the preferredimaging modality for intervention planning, guidance, monitoring, andcontrol. Although x-ray imaging systems for this purpose are widelyavailable, prior-art systems and approaches are significantly limited.In particular, prior art interventional imaging poses the majorimpediments of high subject radiation dose and cumulative physicianexposure to radiation. In certain procedures, the subject x-ray dose maybe high enough to burn the subject's skin. Furthermore, a significantfraction of experienced radiologists and cardiologists are approachingor have reached their annual or life-time accumulated dose limit, andare therefore prevented from, or limited in, the practice of theirskills.

In a typical fluoroscopic procedure, an area detector is used to providea fairly wide imaging field (typically 6 to 16 inches) at a high refreshrate (30 frames per second, or higher). Over the years,image-intensifier technology has evolved to provide electronicamplification and viewing of images. In general, the x-ray image formedon an input phosphor screen is amplified in intensity by a very largefactor, by the electronics of a vacuum envelope within an imageintensifier. The bright, but typically reduced-area output image iselectronically recorded by a video system, and then displayed to thephysician in essentially real time. Recently, a number of vendors haveintroduced digital detectors with refresh rate and x-ray absorptionefficiency comparable to that of the image intensifier. However, theseimprovements have not resolved the issues of high subject and attendantdosage.

Current technologies are further limited, in part, due to use of largearea detectors and large exposure area beams. While a number of systemscurrently offered provide adjustable field-of-view imaging, a largeexposure field is desirable to allow the physician to track the progressof an intervention and to maintain view of specific anatomical landmarksduring a procedure. The requirement for a large exposed area translatesinto high detector costs and the need for a scatter-rejecting Buckygrid, which absorbs about one-half of emitted radiation and thusrequires that the applied dose be increased by a factor of two. Thisadds to the aforementioned high subject and attendant dose; furthermore,the requirement for a large exposed area results in relatively lowrefresh rates over the entire image. For example, read-out of an entirelarge area detector, or a large area of such a detector, limits theimaging refresh rate.

Cardiology and neurology interventions, which typically require theinsertion of a catheter or similar interventional device in thesubject's vasculature, can necessitate continuous or intermittentsubject exposures for extended durations, resulting in high x-ray doses.For example, specific cardiology procedures using current, knowntechnologies, such as in electro-physiology, can last for more than onehour, and accordingly necessitate very high subject doses.Interventional radiologists, cardiologists and other attending staff arealso subject to significant x-ray exposure and dose, to such a degreethat dose limitation regulations may prevent them from active work for asignificant fraction of their available time, thus leading tounderutilization of expensive resources.

Three-dimensional (3D) imaging currently requires complex and expensivesystems. In addition, most currently available 3D imaging systems alsodeliver high subject doses, and often limit access to the subject due touse of a gantry, a large area detector or a combination of areadetectors.

SUMMARY

The methods and system disclosed herein allow for low-dose x-rayexaminations as well as dynamic multispectral x-ray imaging in bothradiographic and fluoroscopic modes, by translating and rotating anarrow-aperture detector and shaping a beam of x-rays accordingly, or bysweeping or rotating a beam of specific shape across the face of an areadetector. Such innovations may facilitate real-time tracking andlow-dose imaging of a catheter tip or other object (e.g., a biopsyneedle or a surgical tool) inserted in a subject during aninterventional procedure, for example as performed in interventionalradiography, interventional neurology and interventional cardiology. Ina more general sense, the disclosed methods and system facilitatereal-time guidance of surgery.

In one embodiment, x-ray examination of a subject or object involvesscanning an x-ray fan-beam of specific shape across the subject orobject. A detector is mounted on a movable assembly below the subjecttable, for example on a detector tray. The detector tray enables (a)independent scanning motions in two (preferably orthogonal) directionsof a plane or other surface, typically chosen to be parallel to asubject table plane, and (b) independent rotation of the detector. Thedetector moves simultaneously along these degrees of freedom.

In one embodiment, a method for dynamic x-ray imaging of a subjectincludes generating an x-ray beam having a non-circular shape about abeam central axis; and irradiating at least part of the subject with thenon-circular beam while rotating the non-circular beam about the beamcentral axis.

In one embodiment, a method for dynamic x-ray imaging of an object orpart of an object includes moving a detector tray supporting a detectorhaving a non-circular shape by rotating the detector tray. An x-ray beamis shaped to generally match the shape or part of the shape of thedetector. The x-ray beam is moved or oriented to track the motion of thenon-circular detector.

A system for dynamic x-ray imaging of an object or part of an objectincludes a gantry to rotate a detector of non-circular shape, and acollimator to shape an x-ray beam to generally match the shape or partof the shape of the non-circular detector. An included beam orientationmechanism tracks the motion of the non-circular detector with the x-raybeam.

The foregoing embodiments may also serve in dynamic, low-dose x-raytomosynthesis and limited angle tomographic imaging systems. In aparticular embodiment, an x-ray source and a detector of a specificshape are moved simultaneously along a number of motion axes, an x-raybeam from the source tracking location and motion of the detector.Multiple images are taken and may be mathematically processed to image aplurality of slices or horizons through a subject. In anotherembodiment, the x-ray source and detector may be stationary, while thesubject is moved along one or more motion axes. One advantage to thistype of tomosynthesis system is that it may be constructed and arrangedfor operation in a plurality of selectable imaging states including atomosynthesis state and a non-tomosynthesis state. The use of a rotatingcollimator and/or rotating detector for low dose imaging permits, forexample, the use of a narrow shaped beam in imaging performed as afluoroscopic system, while also the same system may be operated bydifferent control instructions to provide tomosynthesis.

Still further, the disclosed instrumentalities may be incorporated inmulti-spectral imaging systems with or without computer assisteddiagnosis (CAD). One example of this would be to retrofit the systemthat is shown and described in U.S. Pat. No. 6,950,492, which is herebyincorporated by reference to the same extent as though fully disclosedherein. This type of system, for example, may be provided with arotating collimator assembly as described herein, as well as a rotatingdetector driven in synchronicity with the collimator assembly.

Other objects and advantages of the present disclosure will becomeapparent from the following descriptions, taken in connection with theaccompanying drawings, wherein, by way of illustration and example,embodiments of the present invention are disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top orthogonal view of a system for dynamic low dose x-rayimaging, including a moveable detector assembly.

FIG. 1B is a front orthogonal view of the system of FIG. 1A.

FIG. 1C is a side orthogonal view of the system of FIG. 1A.

FIG. 1D is a geometric perspective view of the system of FIG. 1A.

FIG. 2 schematically illustrates the moveable x-ray detector assemblyshown in FIGS. 1A-1D.

FIG. 3 is a cross-sectional view of a detector tray of the assembly ofFIG. 2.

FIG. 4A illustrates a matrix of detector cells, in accordance with oneembodiment.

FIG. 4B shows the matrix of FIG. 4A, with an arrangement of detectormodules.

FIG. 4C shows the matrix of FIG. 4A, with a alternate arrangement ofdetector modules.

FIG. 4D illustrates the matrix of FIG. 4A, with another alternatearrangement of detector modules.

FIG. 5 is a grid illustrating the relationship between x-ray detectorsamples and image grid points, in accordance with one embodiment.

FIG. 6 illustrates the use of a dynamic, low-dose x-ray imaging systemwith real-time interventional device localization, in accordance withone embodiment.

FIG. 7 is a block diagram showing one embodiment of a system for dynamiclow dose x-ray imaging.

FIG. 8 is a flow chart illustrating data acquisition in a method fordynamic low dose x-ray imaging, in accordance with one embodiment.

FIG. 9 is a flow chart illustrating image processing and reconstructionin a method for dynamic low dose x-ray imaging, in accordance with oneembodiment.

FIG. 10A provides a schematic view of a collimator assembly, inaccordance with one embodiment.

FIG. 10B illustrates adjustment of the shape of the collimator assemblyof FIG. 10A, to reflect the arrangement of detector cells shown in FIG.4B.

FIG. 11 illustrates a system for dynamic low dose x-ray imagingincluding a movable subject table, in accordance with one embodiment.

FIG. 12 is a flow chart illustrating an image acquisition sequence of amethod for dynamic low dose x-ray imaging, in accordance with oneembodiment.

FIG. 13 illustrates parcel or container imaging with a system fordynamic low dose x-ray imaging.

FIG. 14A shows a collimator aperture for use in projecting an x-ray beamof a specific shape onto an x-ray detector, in accordance with oneembodiment.

FIG. 14B shows a collimator aperture for use in projecting an x-ray beanof another specific shape onto an x-ray detector, in accordance with oneembodiment.

FIG. 15A illustrates rotation of an x-ray column with respect to arotation axis, in accordance with one embodiment.

FIG. 15B illustrates motion of an x-ray beam along an axis parallel to along axis of an x-ray tube, in accordance with one embodiment.

FIG. 15C shows rotation of an angle δ of an x-ray column, in accordancewith one embodiment.

DETAILED DESCRIPTION

Before proceeding with the detailed description, it should be noted thatthe matter contained in the following description and/or shown in theaccompanying drawings may be embodied in various forms, and shouldtherefore be interpreted as illustrative, and not in a limiting sense.Elements shown in the drawings are not necessarily to scale and may beexaggerated, enlarged or simplified, to facilitate understanding of theinvention. The system implementation according to the various shownembodiments is amenable to automated controls. These may use circuitryincluding a controller or driver to interface with a computer.Processing may be accomplished using one or more processing unitsoperably coupled with memory and data storage devices. System operationsmay be governed by program instructions and/or circuitry. Actuation, asdescribed below, may be accomplished under motive force provided by stepmotors that are governed by these controls, where such motors areoperably coupled with gears or drive belts to accomplished the desiredmovements. Motive force may alternatively be provided manually, as wellas by pneumatic, hydraulic, or magnetic devices.

In one aspect, servo mechanisms may be governed by feedback control tomaintain alignment between a shaped beam that is emitted through acollimator and a shaped detector. This is particularly useful inembodiments that utilize a rotating collimator and a rotating detectorthat move in synchronicity with one another. In one example of thiswhere the detector is slightly oversize relative to the beam, a detectorsense signal indicating misalignment may result in the detector and/orthe collimator being rotated to restore alignment. Furthermore, thedetector sense signal may be interpolated for projection onto a uniformreference grid so that there is no loss of data.

Turning now to FIG. 1A, a system 100 for dynamic low dose x-ray imagingis shown in a top orthogonal view. System 100 for example allows asignificant reduction in subject and physician dose while permittingeffective performance of an interventional procedure. A frame structure(or cradle) 102, designed for placement beneath or within a subjecttable (see, e.g., table 1110; FIG. 11), permits relative motion of adetector assembly 104 (shown bounded by a circle) with respect to thetable. An x-ray tube apparatus assembly, such as a column or x-raysource assembly having an x-ray tube and a collimator, may for examplebe set upon rails that allow motion of the assembly in a directiongenerally parallel to the subject table.

In one embodiment, a tube column assembly 106 is placed on one side ofcradle 102 and the subject table, and may be dynamically rolled alongrails 108 (or similar translation structure, such as a slide or rollerassembly), which are for example parallel to the subject table, duringthe examination. In one embodiment, an x-ray tube 110 pivots withrespect to a pivot axis (x) 112 that lies generally parallel to thesubject table (alternatively, image plane x O z). The combination ofindependent detector motion (relative to a surface that is often chosento be parallel to the plane of the subject table), tube columntranslation and tube rotation, together with an adjustable collimatorassembly (e.g., assembly 1010, described herein below with respect toFIG. 10), allows projection of an x-ray beam 114 of specific shapetowards any area on the subject table.

FIG. 1B depicts a front view and FIG. 1C depicts a side view of system100. Combined FIGS. 1A-1C show a longitudinal subject axis (z) 116, alateral axis (x) 118 and a table-to-source axis (y) 120. Axis y′ 122(shown in FIG. 3) passes through the detector assembly 104 center ofrotation and is orthogonal to a detector tray, e.g., rotable tray 204,FIG. 2.

FIG. 1D is a perspective view illustrating exemplary geometry of system100. An object or subject axis z′ 124 is for example generally parallelto longitudinal axis 116, and passes through the object or subject'scenter of gravity. In the case of subject imaging, this axis 124 may becollinear with the subject's main axis of elongation. In the case ofinanimate object imaging, the object axis is chosen by convention to beparallel to longitudinal axis 116 and passing through the object'scenter of gravity. The x-ray source, e.g., projection source (S) 126 islocated at a point that does not belong to the chosen object axis, andis retained as defining the vertex of a geometric projection source. Aprojection direction 128 is then defined as the line passing by theprojection source 126 and the object axis 124 and orthogonal to theobject axis 124; projection plane 130 is then defined as the planecontaining object axis 124 and orthogonal to the projection direction128. Sp represents the orthogonal projection of source 126 upon plane130. An x-ray beam as shaped and defined by a collimator assembly,described herein below with respect to FIG. 10, presents at least onedefined main direction, corresponding to the most elongated beamdimension as projected onto the projection plane 130. The intersectionof the elongate beam axis and projection plane 130 defines a line 132 onthe projection plane 130. The aforementioned directions may bedetermined according to a fixed, or laboratory reference system 134.Further, an x-ray beam shaped and defined by a collimator assemblyaccording to the principles laid out herein presents a beam central axis136 passing through the source 126 and generally passing through thegeometric center of the beam projection in plane 130 or in the plane ofdetector assembly 104. In practice, such a beam central axis may bechosen to correspond to the rotational axis of a rotating collimatorassembly, for example as described herein below with respect to FIGS.10A-10B and 14. The beam central axis may also be chosen to pass throughthe detector assembly 104 center of rotation. It is noted that, ingeneral, the beam projection onto plane (P) does not include pointS_(p), nor does the x-ray beam necessarily include projection direction128.

As shown in FIG. 2, system 100 may provide a fast, full-frame samplingdetector assembly, designed according to the principles disclosedherein. Detector assembly 104 for example includes a detector 200 havingdetector cells 202 arranged as a matrix, shown as having a rectangularshape. Detector 200 may mount on a moveable assembly, such as a detectortray 204, itself assembled on sets of rails 206 and 208 underneath orwithin a subject examination table (e.g., table 606, FIG. 6), whichenable independent motion along two directions x and z. As shown in FIG.2, detector tray 204 is circular; however, alternate shapes may beutilized in connection with detector 200.

In the embodiment illustrated in FIG. 2, detector tray 204 has threedegrees of freedom: (1) translation Δz along the longitudinaltable/subject axis 116, (2) translation Δx along the orthogonaldirection 118 in the plane of the subject table, and (3) rotation Δθwith respect to axis y′ 122 (see FIG. 3), which is generally orthogonalto the plane of the subject table and generally in the direction of axis120. These degrees of freedom may be activated independently, incombination, in turn or simultaneously. FIG. 2 illustrates part of themechanical assembly that enables these motions. Detector tray 204 ismounted on an assembly (illustrated as two beams 210 parallel to axis116). Beams 210 terminate at a system of wheels or similar translationstructure, such as a slide or roller assembly (not shown) that allowsmotorized translation along rails 206, parallel to x axis 118. Rails 206also terminate at a system of wheels or similar translation structure,such as a slide or roller assembly (not shown) that rolls on parallelrails 208, oriented parallel to z axis 116. Accordingly, the center orcenter of rotation O′ 212 of detector tray 204 can be juxtaposed withany location within a plane (or upon a surface) that is generallyparallel to the subject table, for example, image plane x O z, shown inFIG. 5 (subject to mechanical limitations on excursion ranges), byactuation of motors for translation along rails 206, 210 (motors notshown). Further, as shown in FIG. 3, detector tray 204 is mounted with arotation axis y′ 122 generally orthogonal to a plane that is locallytangent to the detector motion surface or plane (e.g., image plane x Oz, FIG. 5). Detector tray 204 may freely rotate around axis 122.

FIG. 3 provides a cross-sectional view of detector assembly 104 alongthe line AA′ 214 of FIG. 2. Line AA′ passes through detector center 212and is parallel to the elongate axis of detector 200, e.g., parallel toa plane including detector cells 202. Power is for example provided todetector 200 via a brush link 302 and a slip-ring 304 assembly, althoughit is understood that power provision may be accomplished usingalternate components. Transmission of detector data between rotatingtray 204 and a detector base 306 may be provided by a transmitter andreceiver assembly, for example including transmit and receive elements308, 310, which transmit detector data through radio-frequency (RF)signals. However, other known methods of data transfer, such asbrush-link data transfer, may provide data transmission from rotatingtray 204 to base 306 (and from base 306 to tray 204).

Detector base 306 is a non-rotating base, for example due to fixedmounting on a non-rotating portion of detector assembly 104.Transmit/receive elements 308, 310 may mount with rotating tray 204 andnon-rotating detector base 306, respectively. The above-describedcombination of features facilitates unimpeded and unlimited rotation ofdetector tray 204 for any number of clockwise or counter-clockwiserevolutions generally in the plane of the detector, and further enablestransmission of power and data to and from detector assembly 104.Alternatively, and in other embodiments, detector motions may berestricted to two directions within a plane; or motion may be restrictedto a single scan direction, either within a plane or along a curvedpath.

FIGS. 4A-4D illustrate four of a number of possible detector cellarrangements upon detector tray 204. FIG. 4A shows a matrix of detectorcells 402 arranged along a slot, as elongated rectangular matrix 403. Inone embodiment, detector matrices are designed as a combination ofsquare or rectangular detector modules that can be tiled along anydimension in a plane or surface. Current detector technologies allowdesign of such modules, possibly including backplane read-out; with suchan arrangement, the spacing or gap between adjacent detector modules canbe reduced to a dimension less than or equal to that of the detectorcell pitch.

FIG. 4A also shows the intersection 190 of an x-ray beam (not shown)central axis with the plane (e.g., image plane x O z, FIG. 5) ofdetector assembly 104. FIG. 4B shows rectangular matrix 403 of FIG. 4Awith additional detector modules 404 provided near center of rotation212. Such an arrangement, when properly matched by a source-collimatorassembly, may provide effective trade-offs between the amount of areathat is under continuous x-ray exposure and the areas more distal fromcenter of rotation 212, which are exposed only twice for each fullrotation of detector tray 104.

As shown in FIG. 4C, alternate (or additional) detector module arrays406 may be provided. Arrays 406 for example have various widths, lengthsand detector cell sizes, along with other variable design parameters,and may be generally arranged along a plurality of radial lines passingthrough center of rotation 212. FIG. 4D illustrates the use of fouradditional detector module arrays 408 arranged as lines that are forexample matched to a multi-slot collimator assembly. Many otherarrangements of module arrays 408, such as areas of varying widths,lengths and detector cell resolution, are possible and may be designedto optimize specific performance.

The fast, full-frame sampling detector allows refresh of the part of animage disk, such as a portion of the fixed image grid that is covered bytray 204, that is spatially coincident with a rotating detector atvarious rates. In one example of this, areas near rotation axis 122 maybe refreshed at the intrinsic detector sample rate, while areas towardthe periphery of the disk can be refreshed at a rate that is a functionof detector sample rate, detector angular velocity, and detector cellarrangement. This flexibility facilitates relatively slow refresh of theouter part of an image, while faster refresh is provided at and near theimage center, thus achieving an overall reduction in dose.

In an interventional procedure using a catheter, for example, the fasterrefresh at the image center allows a physician to focus clearly on thecatheter tip. The refresh rate at the image periphery is for examplesufficient to provide landmark data for navigation, while still reducingoverall dose to the patient, physician and any attending staff. Dose isreduced because at any given time, a much smaller total area is exposedto radiation as compared with conventional fluoroscopy procedures.Further, as a Bucky grid is not necessary when using a beam covering areduced area, another two-fold dose reduction may be realized becausethe delivered dose does not have to be increased to compensate for Buckygrid absorption.

An x-ray source filter (not shown) can be shaped to provideproportionally increased flux towards the extremities of the detectormatrix (e.g., detector matrix 403) so that upon continuous rotation, thex-ray dose to various areas of an image disk can be further modulated,by design. In such an embodiment, the filter, located on the tube sideof the imaging chain, provides more x-ray attenuation at the imagecenter, and gradually less attenuation towards the side ends of thex-ray beam. Alternatively, the filter may be made of materials withessentially uniform attenuation properties. In other embodiments, suchas those employing a plurality of detector lines or areas, the filtercan be designed to essentially match the detector shape. Furthermore,different filter properties may be used for each of the correspondingdetector areas or lines, therefore providing for simultaneousmulti-spectral imaging. In a simple example with a detector arraycomprising two orthogonal rectangular cell arrays, one of therectangular detector areas could be illuminated by a high energy beam,while the second rectangular area would be imaged by a low energy beam.

FIG. 5 illustrates the relationship between detector samples 502 andimage grid points 504. In a general case, a given detector samplecontributes to a number of image grid samples in a local neighborhood. Anumber of algorithms have been described, such as the “gridding”algorithm first used in astronomy, that allow efficient interpolationand distribution of the detector samples to the image grid samples. Asis seen from FIG. 5, in a general case, center of rotation 212corresponds to the origin of the referential (x″ O z″) associated withthe moving detector. The origin may be on any point with respect to thetable or image plane (x O z), and rotation angle θ 506 may take anyvalue.

FIG. 6 illustrates the use of a real-time interventional devicelocalization system 600 together with the dynamic x-ray system describedherein above. In one embodiment, three radio-frequencyemitters/receivers 604 (not to scale) are placed adjacent a subjecttable 606 and in such an arrangement as to provide sufficient signalseparation for accurate three-dimensional device tip localization. Asmall assembly 608, for example having three coils, is contained withina device tip (which is for example within region of interest 612, shownbounded by a dotted circle). Analysis of the signals thus received atone or a multiplicity of frequencies permits accurate, real-timelocalization of the device tip with respect to the table coordinatesystem, as known in the art. This localization information is fed-backin real-time to the dynamic x-ray imaging system (e.g., system 100), andadjustments are dynamically made to the detector tray position (e.g.,tray 104 position), x-ray tube column position (e.g., column 106position), x-ray tube angle (e.g., tube 110 angle), and x-ray collimatorassembly position (e.g., assembly 1010 position), as necessary to trackthe progress of the device tip with a detected x-ray beam.

Further adjustments to detector raster mode, location and rotation maybe made as necessary to enable dynamic tracking of the device tip.Scanning modes include translation of the detector along either thedirection parallel to or the direction orthogonal to the short axis ofthe detector cell matrix (e.g., in directions z″ or x″ of FIG. 5);combinations of parallel and orthogonal translations; rotation of thedetector with respect to center of rotation axis 122, and combinationsof rotation and translation motions of the detector tray within theplane of the detector assembly.

In a specific imaging mode, and for illustration, it might be desirableto ensure that the center 212 of detector tray 204 is always positionedwith respect to table 606 and the system x-ray tube (e.g., tube 110) insuch a manner that the x-ray shadow of the device tip projects ontocenter 212. Tracking may rely on automatic device tip detection in theprojection image, motion of the detector assembly and/or motions of thex-ray imaging chain. A dynamic fluoroscopic image is obtained bysimultaneously rotating tray 204 with respect to its instantaneouscenter, rotating the collimator assembly in synchronicity with therotating tray 204, and/or translating and/or rotating the collimatoracross the x-ray port and/or rotating the x-ray tube, for acquiringx-ray data. Synchronicity between the tray and the collimator may beprovided mechanically, by design, or through a feedback loop and sensingof relative motion of the x-ray beam with respect to the detector, forexample feedback from pixels radiated by the beam penumbra may indicatemisalignment between the tray and collimator.

Other applications and modes of implementation are also possible. X-raydata acquisition may be performed in either a pulsed or a continuousmode. In addition, other device or object localization systems orinstruments may be employed with system 100.

FIG. 7 presents a block diagram showing system components. The systemcomprises an image acquisition and review workstation 710, an interfaceto an hospital or imaging network 730, and a gantry 740. The imageacquisition and review workstation 710 has an acquisition workstation712 with a graphical user interface (UIF) 713; a controller 714 whichreceives inputs from external devices such as an EKG and/or a devicelocalization system, and drives x-ray emission, acquisition, systemmotions, and tracking; a data-preprocessing computer architecture 716for data calibrations and corrections; an image reconstruction anddecomposition engine 718; an image display 720; and, for specificapplication, a feature detection and characterization engine 722interfacing to a database 724.

The gantry 740 includes: a high-voltage generator and inverter 742 forthe generation of time-varying kVp and mA waveforms; a controller 744for the selection of an anode track and control of x-ray sources andx-ray focal spot parameters (the selection and control of which may varyas a function of time); a controller 746 for the activation of x-rayfilters and collimation devices (such action also variable as a functionof time); a motion control architecture 748 which itself comprises acontroller 750 for the subject, detector cradle, and x-ray tube columnpositioning and a controller 752 for the motions of the detectorassembly (e.g., along z, x axes 118, 116 and rotation Δθ about axis122).

FIG. 8 shows an exemplary data acquisition sequence 800. Followingstartup of the sequence, the user provides input information relating tothe subject/object to be imaged, and type of data acquisition sequenceto be performed, in step 802. Step 802 for example includes selection ofacquisition parameters and input of body information at graphical UIF713, of FIG. 7. The subject/object is then positioned, in step 804. Theacquisition sequence and EKG and device localization feedback areinitialized in step 806. Feedback from an EKG and/or feedback fromdevice localization inputs may occur in essentially real time, or dataacquisition may proceed according to a pre-determined imaging sequence.

The synchronized acquisition sequence (indicated by dotted box 808)controls selection of spectra, including selection of: an anode track,in step 810; a focal spot geometry, in step 812; x-ray techniques, forexample selection of KVp or mA waveforms, in step 814, and filtration,in step 816. The x-ray source and detector assembly are set in motion,in step 818, for example to perform a specific series of imageacquisition sequence, to generate a particular imaging raster, or todynamically track an interventional device tip such as a catheter,sheath, guide wire or other interventional object such as a biopsyneedle or a radiation seed implant, e.g., in brachytherapy applications.Collimator controls are activated to dynamically track the detectorlocation and orientation or to perform a specified acquisition sequence,in step 820. Collimator controls are for example activated as a functionof body position of a subject upon a subject table, and/or otheracquisition parameters. Data is acquired, in step 821, and calibrationdata is gathered, in step 822. Acquired data (gathered in step 821) ispre-processed, using the calibration data, in step 824. Calibrations andpre-processing is for example performed by computer architecture 716.

An image reconstruction model is generated, in step 826, and an imagedecomposition model is generated, in step 828. Image reconstruction anddecomposition is performed using the reconstruction and decompositionmodels of steps 826, 828, for example by image reconstruction anddecomposition engine 718, in step 830. As may be desirable, the imagedata are automatically analyzed by a CAD engine, in step 832. The CADengine, e.g., characterization engine 722, for example providesautomatic detection, characterization, and classification of features bydata processing and/or by accessing a database, in step 834. Acquiredimages are then displayed, in step 836, and reviewed, in step 838. Auser for example provides inputs via UIF 713 as necessary, for imagereview.

FIG. 9 details a method 900 for image reconstruction using an imagereconstruction algorithm, e.g., as performed by engine 718, FIG. 7. Rawprojection data are re-ordered and interpolated (if necessary dependingon the specifics of the data acquisition sequence), in step 902.Following pre-processing of projection data, in step 904, the algorithmproceeds to image generation. Image generation commences with adetermination of whether or not dynamic multispectral x-ray imaging(DXMI) is used, decision 906. In the case of a DMXI image acquisitionsequence, a stack of multispectral projection images is obtained, instep 908. These images are decomposed using either a matrix inversionapproach, step 910, or an iterative approach, step 912. A stack ofdecomposed projection images is acquired (via step 910 or 912), in step914. The acquired images are then used to refresh the full field image,in step 916, thus providing a fluoroscopy or radiography image sequence,step 918.

Alternatively, when DMXI is not used according to decision 906, afurther case differentiation is made depending upon rotation of thedetector tray, in decision 920. If the detector tray is not rotating,but translating (with the detector tray in an arbitrary angle), varioussimpler interpolation algorithms may be employed to dynamically buildand refresh the full field image, e.g., by interpolating the detectorraster to image grid, in step 922. The full field image is refreshed,step 916, and the fluoroscopic or radiographic image sequence generated,in step 918. Alternately, if the detector tray is rotating, decision920, a more complex interpolation algorithm, such as a griddingalgorithm, is employed, in step 924, for image field refresh, step 916,and generation of a fluoroscopic or radiographic image sequence, step918. The choice of x-ray techniques as well as image frame ratecontributes to the distinction between fluoroscopic and radiographicsequences.

FIG. 10A schematically shows one embodiment of a collimator assembly1010. A collimator 1020 is mounted on a rotatable ring 1022. Such ringcan also be translated along an axis x″ 1024, by rolling along two rails1026 parallel to x″. A system of collimator blades such as independentlyadjustable sets of blades 1028 and 1030 mount with the rotatable part ofthe collimator. Collimator blades 1028 and 1030 open or close alongtheir respective axes, for example to effect an aperture 1032 and anaperture 1032 location that allow a narrow x-ray beam to be generatedand to project onto a detector (e.g., detector assembly 104) for anyposition and/or orientation of the detector. A beam of specific shape,such as a beam including a number of fans (with or without a centralwide area), may also be defined by blades 1028 and 1030 as subjected tosuitable modifications (not shown). The shaped beam is then translatedor rotated across the face of a large area detector or in synchronicitywith the motion of a detector of specific shape.

In one embodiment, the shape of the x-ray beam is adjusted to reflect aspecific arrangement of detector cells. FIG. 10B shows an aperture 1050shaped to match the arrangement of cells 402, 404 in FIG. 4B. Aperture1050 is defined for example by an aperture plate 1052; however, aperture1050 may also be defined by combining plate 1052 with blades 1028, 1030.Aperture 1050 size and shape may thus be controlled by any one of blades1028, blades 1030 and aperture plate 1052, or by any combination ofblades 1028, 1030 and plate 1052.

To allow a continuous rotation mode, power is provided to the rotatablepart of collimator assembly 1010, for example through a slip-ring andbrush design (see, e.g., brush link 302 and a slip-ring 304 assembly,FIG. 3). Collimator assembly 1010 includes a filter (not shown), whichmay include filter materials of various attenuation and x-ray energyfiltration properties. In one embodiment, filter attenuation varies fromthe center to the sides of collimator 1020; in another embodiment,different filters may be provided for each of the different areas of thecollimator aperture 1050. Simplified collimator embodiments are alsoprovided by use of an aperture plate 1052, providing for beam formationin a shape matching that of the detector cells such that variations inthe projection imaging geometry during an imaging sequence are accountedfor. For example, plate 1052 may ensure that a maximum beam widthremains less than the width of the associated active detector array. Thecombination of x-ray aperture, collimator and filter shapes the x-raybeam both spatially and spectrally, as is known in the art. Shaping mayalso be accomplished using only an aperture and a filter, a collimatorand a filter, or a lone collimator.

Dynamically adjustable blades 1028, 1030 may provide a beam of specificwidth that in typical operation always projects onto the x-ray detector.Further, the width and position of the x-ray beam with respect to thedetector matrix may be dynamically adjusted during the image acquisitionsequence. This width adjustment, in particular, provides further controlof the x-ray dose and noise properties of the image. Analysis of thefull-frame data enables tracking of the beam umbra and penumbra onto theactive detector. The location and orientation of the beam with respectto the detector as a function of time, for example, allows computerized,automatic prediction of necessary imaging chain (x-ray tube, collimator,detector) adjustments to either track the detector for a given imagingsequence, and/or to track a moving point such as the tip of aninterventional device.

Accordingly, detector motion may be tracked in real time based ondetected x-ray profile information and/or instantaneous detectorcoordinate information. Tracking is for example achieved by acombination of the following motions: relative advance of the subjecttable with respect to the x-ray column, either by table motion or x-raycolumn motion; x-ray tube pivoting; collimator translations, for examplealong pivot axis 112, FIG. 1A and/or along an axis generally orthogonalto axis 112 (not shown); collimator rotation, and collimator bladesadjustment (both with respect to the width and length of thecollimator-defined aperture). In one embodiment, a specific raster androtation sequence is programmed into both the detector tray controls andinto the x-ray tube and collimator assembly controls.

By these instrumentalities, the x-ray beam is spatially shaped to matchat least part of the moving detector, in the sense that part of theactive detector is illuminated by the beam umbra (largest intensity),and the beam penumbra (or area of drop in intensity) is also imaged bythe active detector. For a detector of given shape, the collimator maybe adjusted such that the x-ray beam illuminates only part of thedetector, For example, if a detector has an elongated array withadditional cells along a second, generally elongated array, thecollimator may be adjusted such that the x-ray beam illuminates only oneof the two elongated arrays. Many other geometries are possible for boththe detector and the collimator, as guided by the choice of x-rayimaging sequence and application type.

FIG. 11 illustrates one embodiment of a system 1100 for dynamic,low-dose x-ray imaging utilizing an additional degree of freedom. Asubject-supporting table 1110 is adjustable via mechanical actuators1112, allowing positioning of table 1110 with respect to detector cradle102. This feature provides for variable geometry and variablemagnification of a subject onto a detector plane. Both cradle 102 andsubject table 1110 elevation along axis 120 can be adjusted via a cradlesupport 1114 and corresponding actuators (not shown).

In one embodied image acquisition mode, both dynamic multi-spectralx-ray imaging (DMXI) acquisition and detector tray rotation occursimultaneously. FIG. 12 illustrates a corresponding method for imageacquisition. Method 1200 is for example governed by an image acquisitionalgorithm. Each array or array cell may be subject to variable timing,depending on the specifics of a given acquisition sequence as well aslocation within the detector. Such variable timing may include bothoffsets and sample times. Thus, detector timing sequences aredetermined, in step 1210. Data read-out for each column of the utilizeddetector array is independently determined, based on image acquisitionsequence parameters and detector rotation, in step 1220. Gridding imagereconstruction (or similar interpolation method) is performed for theacquired frames, in step 1230. A stack of multispectral projectionimages is generated, in step 1240. These projection images are input toa decomposition algorithm that performs either a matrix inversion orsimilar analytical decomposition (e.g., SVD regularization), in step1250, or an iterative inversion, in step 1260. Accordingly, a stack ofdecomposed projection images is generated, in step 1270. The acquireddecomposed images are then used to refresh the full field image, in step1280, and to generate a fluoroscopy or radiography image sequence, instep 1290.

FIG. 13 illustrates use of a system for dynamic, low-dose x-ray imagingfor imaging of parcels, inspection of parts, imaging of containers andthe like. In one embodiment, system 1300 includes a conveyor belt 1310for transporting a parcel 1320 to be imaged. Acquisition of amultiplicity of projections of the same object may be facilitated bysimultaneous translation of the x-ray tube column along, for example, zaxis 116.

FIGS. 14A-14B depict a collimator assembly 1410 with collimatorapertures 1420A, 1420B and collimator blades (not shown), shapes a beamto a specific shape 1430A, 1430B as projected onto a detector 1440. Thebeam 1450 is scanned and/or rotated across the face of detector 1430,which is for example a two-dimensional detector. Data is read out ofdetector 1440 either in a raster fashion or, as possible with newertechnologies such as CMOS design, read-out of pre-determined areas in aspecific sequence. FIGS. 14A and 14B present two embodiments for twospecific x-ray beam shapes.

FIGS. 15A-15C schematically present a set of detector motions withrespect to a number of axes. Tomosynthesis or limited-angle tomographicimaging may be enabled by moving a detector of specific shape alongmotion axes of FIGS. 15A-15C, while simultaneously moving the x-raysource along motion axes of FIGS. 15A-15C and shaping an x-ray beam totrack the location and motion of the detector. Alternately, one or bothof the x-ray source and detector may remain stationary during all orpart of the imaging sequence. Where both the source and detector arestationary, an object to be imaged may move through an x-ray beamemitting from source to detector. For example, an object such as asuitcase moving along a conveyor belt may pass through the beamgenerated by stationary column assembly 106. Such an embodiment may findparticular use in security screening applications, such as airportsecurity or customs inspections.

In particular, FIG. 15A illustrates rotation of x-ray tube columnassembly 106 by an angle φ 1510 with respect to rotation axis y′ 1515shown in FIG. 15B. FIG. 15B in turn presents motion of tube columnassembly 106 along axis x′ 1520 essentially including the x-ray tube 110long axis. A displacement Δx′ 1530 enables acquisition of a multiplicityof projections at various angles. Column assembly 106 (or x-ray tube110) may rotate to track motion of an associated detector, e.g.,detector 200. Alternately, column assembly 106 may remain stationarywhile detector 200 rotates, for example upon tray 204. In anotherembodiment, for example as described above with respect to airportsecurity, above, both column assembly 106 and detector 200 center 212remain stationary while a subject or object to be imaged moves through arotating x-ray beam.

FIG. 15C illustrates rotation of angle δ 1540 of column assembly 106.This motion may also enable acquisition of a multiplicity of projectionsat various view angles; a similar effect may be enabled by translatingx-ray tube 110 along z axis 116, as might be possible through rolling ofthe entire column 106 in this direction.

Operation

In one embodied mode of operation, the detector performs an initial“scout” scan of either the entire table or of a sub-area as prescribedby the user. Based on this initial scout image, the user or the systemcomputer prescribes an area to be imaged. A number of imaging modes arepossible, including linear raster scan of the area (possibly including amultiplicity of raster scan lines), or a combination of translation ofthe detector tray rotation axis O′ together with continuous rotation ofthe detector around rotation axis y′.

In a second embodied mode of operation, the system is set to track theprogress of an interventional tool, such as the tip of a catheter orother interventional device. The system automatically selects thedetector tray center O′ position so that the projection of the devicetip is superimposed with the detector center O′. These automatic motionsmay be achieved either with or without simultaneous x-ray columntranslation along the subject table and associated x-ray tube pivotingwith respect to axis x′, depending on the imaging mode selected. Oncethe device tip has reached the theatre of operation (such as thecoronary arteries in a cardiology procedure), the detector continuesrotation around O′, while minor adjustments to the O′ location aredynamically made to maintain the device tip at image or detector traycenter. The point O′ may also be held at a given position, whilefluoroscopic image refresh occurs through the continuous acquisition ofdata by the rotating detector. Alternatively, the detector trajectorymay not include rotation, but be limited to a sequence of scans alongspecific raster lines, with the detector main axis either orthogonal orat a non-90-degrees angle with respect to the scan direction.

In another embodied mode of operation, adjustment of various systemparameters, including relative position of the x-ray tube apparatus withrespect to the object and detector, allow dynamically acquisition ofseveral images of the same object for various projection angles andprojection geometries. The projections may be chosen dynamically by theuser, or the system may automatically loop through a pre-determinedsequence of projections.

Dynamic operation of the system may also enable acquisition of imagedata at different levels of image noise, spatial resolution or spectralcomposition. In particular, the system may first be operated at a firstlevel of resolution, noise, spectral composition or other imagingparameter, and then be switched to a higher resolution or reduced noisemode or different spectral composition, for example upon user orautomatic detection of an abnormality or threat. Imaging acquisition mayalso take place at various levels of resolution, either dynamically intime or spatially; such various resolution and noise levels being forinstance achieved through variable binning of the native resolutiondetector pixels. The various detector lines or arrays may have variablenative pixel resolution as a function of the line or as a function ofdistance from detector iso-center.

The present instrumentalities may be applied to the operation offlat-panels area detectors, either specifically designed or operated asfollows: A beam of specific shape and spectral characteristics is sweptacross the active surface of the detector. In one embodiment, a fan-beamis scanned linearly and/or rotated using a collimator assembly andmethods according to the principles described herein. In anotherembodiment, a beam of more complex shape (such as may be formed by acollimator plate as described above) and/or containing several fans anda central area, is scanned linearly and/or rotated. The flat-panel dataare read out to allow image formation and image refresh at ratesdepending on the spatial location of a given image pixel.

The present instrumentalities further apply to the operation of computedtomography (CT) systems, either specifically designed or operated asfollows: A beam of specific shape and spectral characteristics is sweptacross the active surface of the detector, as described in the paragraphabove. The beam sweeping may occur independently or simultaneously withgantry rotation.

In all embodiments where the detected x-ray beam is under-collimatedwith respect to the active detector, detector cells that are not exposedby the primary beam or by the beam penumbra detect scattered radiation.These measurements may be leveraged to perform scatter correction and orfurther object analysis and characterization.

Full-frame sampling of the active detector allows closed-loop dynamicadjustments to the x-ray beam parameters, including peak kilo-voltage,tube current, tube target location and selection, and filtration, toadapt x-ray imaging parameters to the composition of the object oranatomy being imaged.

In a tomosynthesis or limited-angle tomographic imaging mode, the x-raysource is set in motion along at least one of axes z 116 (columnrolling), x′ 1520 (tube translation along tube main axis), rotation ofangle φ 1510 with respect to y′ 122, or rotation of angle δ, 1540, orany motion that similarly contributes to the acquisition of amultiplicity of views (or projections) of the object to be imaged. Giventhe dynamics of the x-ray tube and the dynamics of the detector, thetracking algorithm orients tube angle (rotation with respect to the axisx′) and/or collimator position and orientation, such that an x-ray beamof the appropriate shape projects onto the active part of the detector.This sequence of data acquisition results in the obtaining of amultiplicity of projection data that are then input to the 3D imagereconstruction algorithm. A 3D image sequence can then be refreshed withthe newly reconstructed information. Thus the system is designed for afourth-dimensional data acquisition (time varying 3D data sets).

The advantages of the above described apparatus embodiments,improvements, and methods should be readily apparent to one skilled inthe art, as to enabling the design of low-dose dynamic x-ray imagingsystems and low-dose tomosynthesis and limited-angle computedtomography. Additional design considerations may be incorporated withoutdeparting from the spirit and scope of the invention. It should thus benoted that the matter contained in the above description and/or shown inthe accompanying drawings should be interpreted as illustrative and notin a limiting sense. Accordingly, the following claims are intended tocover all generic and specific features described herein, as well as allstatements of the scope of the present methods, and systems which, as amatter of language, might be said to fall therebetween.

1. A method for x-ray tomosynthesis or limited-angle CT of a subject,comprising: generating an x-ray beam having a non-circular shape about abeam central axis; irradiating at least part of the subject with thebeam while rotating the beam about the beam central axis; and changingthe x-ray beam source position with respect to the subject duringirradiation to acquire at least two subject projections.
 2. The methodof claim 1, further comprising linearly translating the beam across atleast part of the subject.
 3. The method of claim 1, wherein the x-raybeam is shaped to match at least part of a moving detector.
 4. Themethod of claim 3, wherein the shaped beam tracks the detector.
 5. Themethod of claim 1, wherein the beam central axis is generally alignedwith the center of a subject region of interest and the beam rotatesaround the beam central axis.
 6. A method for x-ray tomosynthesis orlimited-angle CT of a subject or part of a subject, comprising: moving adetector tray supporting a detector having a non-circular shape byrotating the detector tray; shaping an x-ray beam to generally match theshape or part of the shape of detector; moving or orienting the beam totrack the motion of detector; and moving the x-ray beam source withrespect to the subject, while moving the beam to track the detector, toacquire at least two subject projections.
 7. The method of claim 6,further comprising translating the detector tray.
 8. A system for x-raytomosynthesis of limited-angle CT of a subject or part of a subject,comprising: a gantry to rotate an x-ray detector of non-circular shape;a collimator to shape an x-ray beam to generally match the shape or partof the shape of the x-ray detector; a beam orientation mechanism totrack the motion of the detector with the x-ray beam; and a x-ray beamsource gantry to move an x-ray beam source with respect to the subjectduring tomosynthesis or limited-angle CT.
 9. The system of claim 8,wherein the detector gantry translates within a plane or curved surface.10. The system of claim 8, comprising means for unlimited rotation ofthe x-ray detector.
 11. The system of claim 10, comprising means forpower and signal transmission between the rotating detector and anon-rotating supporting assembly.
 12. The system of claim 8, wherein thedetector of non-circular shape is mounted on a rotatable tray, therotatable tray mounted on a non-rotatable detector assembly that ismoveable along at least two dimensions of a surface.
 13. The system ofclaim 12, wherein the non-rotatable detector assembly comprises aslip-ring assembly for transmitting power to the rotatable detector traywhile not limiting the number of detector rotations.
 14. The system ofclaim 12, wherein the detector assembly comprises a wireless transmitterand receiver for transmission from the rotatable detector tray to thenon-rotatable detector.
 15. The system of claim 8, wherein the detectorcells are arranged on a rectangular elongated matrix.
 16. The system ofclaim 8, wherein the detector cells are arranged on a rectangularelongated matrix and wherein additional detector cells are provided ator near the detector tray rotation center to provide for a larger x-raydetection area near the axis of rotation.
 17. The system of claim 8,wherein the detector cells are arranged on a rectilinear elongatedmatrix and wherein additional detector cells are provided on one orseveral additional matrices.
 18. The system of claim 8, furthercomprising means to dynamically and automatically track the location ofa point in space.
 19. The system of claim 18, wherein the point in spacecorresponds to a point on an interventional medical device selected fromthe group of a catheter, sheath, brachytherapy seed, probe, ablationelement, and guide wire.
 20. The system of claim 8, wherein thecollimator is rotatable around an axis.
 21. The system of claim 8,comprising means for rotating the x-ray detector in synchronicity withthe collimator.
 22. The system of claim 8, wherein the system isconstructed and arranged for operation in a plurality of selectableimaging states including a non-tomosynthesis state.